Abstract

Background

Growth rate In children is reported to have seasonal variability. There are fewer
published data regarding seasonal variability while on growth hormone (GH) therapy,
and none analyzing growth rate with respect to number of daylight hours.

Methods

We analyzed 11,587 3-month intervals in 2277 prepubertal children (boys ages 3–14
years, girls ages 3–12 years) with idiopathic GH deficiency from the National Cooperative
Growth Study (NCGS) database. All were naive to recombinant human GH (rhGH) therapy.
Data were submitted from 31 US study centers. Seasonal variation in height velocity
(HV) was assumed to be associated with the average number of daylight hours during
the interval of time over which HV was computed. Number of daylight hours was determined
from the date of the measurement and the latitude of the study center. Other independent
variables evaluated included: height standard deviation score (SDS) at the beginning
of the interval, chronologic age at the beginning of the interval, time from the start
of rhGH treatment to the middle of the interval, month of the year, body mass index
SDS at the beginning of the interval, rhGH dose/kg, mother’s height SDS, father’s
height SDS, and log base 10 of the maximum stimulated GH concentration.

Results

All variables examined, except month of the year, correlated significantly with interval
HV. There was significant “seasonal” variability at all latitudes, with summer annualized
HV being greater than winter HV. This difference was greatest in the first year of
therapy (0.146 cm/yr/daylight hour; P < 0.0001) but persisted in subsequent years (0.121 cm/yr/daylight hr; P < 0.0001). The difference increased with distance from the equator. Growth rate over
the year was not different among the latitudes reflected in this North American study.

Conclusions

There is “seasonal” variation in growth of children on rhGH therapy that correlates
with number of daylight hours. The effect is modest and is greatest in the first year
of therapy. Annual growth rate appears to be equal in children among latitudes covered
by the US consistent with exposure to an equal number of daylight hours over the year.
The physiologic mechanism behind this seasonal variation is not yet understood.